1. Field of the Invention
The present invention relates to a method for manufacturing an impregnated cathode wherein an impregnated pallet is fixedly fitted in a cathode cup, and more particularly to a method for manufacturing an impregnated cathode wherein upon a process of impregnating an electron emitting material in a porous pallet to produce an impregnated pallet, fixing of the impregnated pallet to a cathode cup is achieved by an oxidation reaction between the electron emitting material and an oxidative material of the cathode cup.
2. Description of the Prior Art
Generally, impregnated cathodes have been used in oscilloscopes which require high current density. Recently, they have been also applied to electron tubes which are used in televisions, since the electron tubes require high resolution and large screen in televisions.
Referring to FIG. 1, there is shown an example of general impregnated cathode constructions. As shown in the drawing, the cathode comprises a cylindrical cathode cup 2 closed at its lower end and made of a high-resistant material, such as molybdenum (Mo). An impregnated pallet 1 is fixedly fitted in the cathode cup 2. The impregnated pallet 1 is made by impregnating an electron emitting material in a porous pallet of a heat-resistant metal such as tungsten (W). The cathode also comprises a cylindrical cathode sleeve 3 made of a high heat-resistant material such as molybdenum (Mo). The cathode sleeve 3 receives the cathode cup 2 in its upper end. Within the cathode sleeve 3, a heater 4 adapted to heat the cathode is disposed at the lower portion of cathode sleeve 3.
The impregnated cathode with the above-mentioned construction is disposed in position within an electron gun of electron tube. In operation, as a drive power is applied to the heater 4 disposed in the cathode sleeve 3, the heater 4 generates heat. According to the heating of heater 4, heat is accumulated in the cathode sleeve 3 and then transferred to the cathode cup 2. The transferred heat to the cathode cup 2 is then transmitted to the impregnated pallet 1, so that the impregnated pallet 1 emits electrons, by virtue of the transmitted heat.
In manufacturing such a general impregnated cathode, the electron emitting material is conventionally prepared by mixing BaO and CaO obtained by decomposing BaCO.sub.3 and CaCO.sub.3 at high temperature, with Al.sub.2 O.sub.3. Such type of electron emitting material is melted and impregnated in pores of a porous pallet under a predetermined impregnation atmosphere, so as to form the impregnated pallet 1. As the impregnation atmosphere, a vacuum or inert gas atmosphere maintained at a temperature of about 1,600.degree. C. is used.
After completing the preparation of impregnated pallet 1, a process for fixedly fitting the cathode cup 2 in the impregnated pallet 1 is performed. As the process, there has been used a method comprising the steps of providing a metal material 5, which is an alloy of molybdenum (Mo) and ruthenium (Ru) or a brazing metal, between the inner closed bottom surface of cathode cup 2 and the impregnated pallet 1 fitting into the cathode cup 2, and then carrying out a brazing at a high temperature.
After completing the fitting, the cathode cup 2 is fixedly fitted in the upper end of cathode sleeve 3 such that its outer peripheral surface is in tight contact with the inner peripheral surface of the upper end of cathode sleeve 3. Thereafter, the heater 4 is inserted into the lower portion of cathode sleeve 3. Thus, the above-mentioned cathode construction is obtained.
However, this conventional method, wherein a brazing at high temperature is carried out to bond the impregnated pallet 1 to the cathode cup 2 under the condition that the metal material 5 is filled between the impregnated pallet 1 and cathode cup 2, has a disadvantage of an increase in manufacturing cost, since the material 5 which is a brazing metal or alloy is expensive.